Antigen-specific T cell responses require multiple interactions between cell surface receptors on T cells and ligands on antigen presenting cells. The primary interaction is between the T cell receptor/CD3 complex and a major histocompatibility complex molecule, which presents an antigenic peptide to the T cell receptor, thereby triggering an antigen-specific signal in the T cells. In addition to this antigen specific signal, T cell responses require a second, costimulatory signal. A costimulatory signal can be generated in T cells by stimulation of T cells through a cell surface receptor CD28 (Harding, F. A. (1992) Nature 356:607-609). Ligands for CD28 have been identified on antigen presenting cells (APCs). CD28 ligands include members of the B7 family of proteins, such as B7-1 and B7-2 (Freedman, A. S. et al. (1987) J. Immunol. 137:3260-3267; Freeman, G. J. et al. (1989) J Immunol. 143:2714-2722; Freeman, G. J. et al. (1991) J. Exp. Med. 174:625-631; Freeman, G. J. et al. (1993) Science 262:909-911; Azuma, M. et al. (1993) Nature 366:76-79; Freeman, G. J. et al. (1993) J. Exp. Med. 178:2185-2192). The B7 proteins have also been shown to bind another surface receptor on T cells related to CD28, termed CTLA4, although the role of CTLA4 in costimulation is still unclear (Linsley, P. S. (1991) J. Exp. Med. 174:561-569).
It has been demonstrated that delivery of an antigen-specific signal to a T cell in the absence of a costimulatory signal does not induce a T cell response, but rather induces a state of unresponsiveness, also termed T cell anergy (see Schwartz, R. H. (1990) Science 248:1349; Jenkins, M. K. et al. (1988) J. Immunol. 140:3324). Based upon this phenomenon, therapeutic approaches for inducing T cell unresponsiveness to an antigen have been proposed which involve the blocking of a costimulatory signal in T cells. For example, a CTLA4Ig fusion protein, which binds to both B7-1 and B7-2 and blocks their interaction with CD28, has been used to inhibit rejection of allogeneic and xenogeneic grafts (see e.g., Turka, L. A. et al. (1992) Proc. Natl. Acad. Sci. USA 89:11102-11105; Lenschow, D. J. et al. (1992) Science 257:789-792). Alternatively, therapeutic approaches have also been proposed for stimulating a T cell response to an antigen on a cell (e.g., a tumor cell). For example, tumor cells modified to express the CD28 ligand B7-1 on their surface have been found to trigger a costimulatory signal in T cells (see e.g., Chen, L. et al. (1992) Cell 71:1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259:368-370; and Baskar, S. et al. (1993) Proc. Natl. Acad. Sci. USA 90:5687-5690).
In addition to antigen-specific and costimulatory interactions, many other cell surface receptors on T cells are thought to serve an accessory function in T cell activation (reviewed in Clark, E. A. and Ledbetter, J. A. (1994) Nature 367:425-428). Examples of such cell surface receptors include: CD4, which interacts with MHC class II antigens; CD8, which interacts with MHC Class I antigens; CD40, which interacts with CD40L (gp39); ICAM-1, which interacts with LFA-1; and CD2 which interacts with LFA-3 (also known as CD58) (Selvaraj, P. et al. (1987) Nature 326:400-403). CD2 has been shown to also interact with CD48 and CD59, although with lower affinity than with LFA-3 in humans (Arulanandam, A. R. et al. (1993) J. Exp. Med. 177:1439-1450; Sandrin, M. S. et al. (1993) J. Immunol. 151:4606-4613).
CD2 is a glycoprotein with a relative molecule mass of 50,000-58,000 which is expressed on thymocytes and mature T cells. CD2 binds to sheep erythrocytes, a property responsible for the phenomenon of T cell E-rosetting. The interaction between CD2 on a T cell and LFA-3 on an APC can facilitate antigen recognition by T cells, thereby stimulating antigen-specific T cell responses (see e.g., Bierer, B. et al. (1988) J. Exp. Med. 168:1145; Moingeon, P., et al. (1989) Nature 339:312; Koyasu, S. et al. (1990) Proc. Natl. Acad. Sci. USA 87:2603; Selvaraj, P. et al. (1987) Nature 326:400; and Bierer, B. et al. (1988) J. Immunol. 140:3358). This effect has been attributed, at least in part, to increased adhesion between the T cell and the APC, mediated by the CD2/LFA-3 interaction. Additionally, it has been demonstrated that T cells in vitro can be stimulated to proliferate and secrete IL-2 in the absence of APCs using an appropriate combination of anti-CD2 antibodies (see e.g., Meuer, S. et al. (1984) Cell 36:897; Feterson, A. et al. (1987) Nature 329:842; Yang, Y. S. et al. (1986) J. Immunol. 137:1097; and S. C. Meuer, in Leucocyte Typing IV, White cell differentiation antigens, W. Knapp et al, Eds (Oxford, 1989) p. 270). For example, T cells can be activated with the anti-CD2 antibody T11.3 in combination with either one of two other anti-CD2 antibodies, T11.2 or T11.1. It has been shown that the T11.3 epitope is a xe2x80x9cneo-epitopexe2x80x9d, which is not exposed on CD2 on resting T cells but is exposed on CD2 upon T cell activation (Meuer, S. et al. (1984) Cell 36:897). Although in vitro culture experiments involving CD2 have implicated this surface receptor in T cell-APC adhesion and T cell activation, it is unknown from these studies what physiological role CD2 may serve in T cell responses to antigens.
The invention pertains to methods for modulating antigen-specific T cell unresponsiveness. The invention encompasses methods for either maintaining or reversing T cell unresponsiveness by inhibiting or stimulating an unresponsive T cell through a cell surface receptor. It has been discovered that antigen-specific T cells which have been rendered unresponsive to an antigen can regain the ability to respond to the antigen by stimulating the T cells through a cell surface receptor, such as CD2. Accordingly, the invention discloses a functional role for CD2 in reversing antigen-specific T cell unresponsiveness (also referred to as T cell anergy).
One embodiment of the invention involves maintaining T cell anergy by contacting anergized T cells with an agent which inhibits stimulation of the T cells through CD2. CD2 inhibitory agents include agents which inhibit an interaction between CD2 and a CD2 ligand (e.g., LFA-3, CD48 or CD59). Such agents include blocking antibodies, soluble forms of CD2 and CD2 ligands, peptides and small molecules. Alternatively, a CD2 inhibitory agent can act intracellularly to inhibit an intracellular signal triggered in the T cell through CD2. Another embodiment of the invention involves reversing T cell anergy by contacting anergized T cells with an agent which stimulates the T cells through CD2. CD2 stimulatory agents include a cell which expresses a CD2 ligand on its surface (e.g., LFA-3, CD48 or CD59), mulivalent forms of a CD2 ligand and stimulatory anti-CD2 antibodies. Alternatively, a CD2 stimulatory agent can act intracellularly to trigger a signal through CD2.
The methods of the invention are useful therapeutically in situations where it is desirable to modulate antigen-specific immune responses, e.g., maintain antigen-specific T cell unresponsiveness or restore antigen-specific T cell responsiveness. For example, it may be necessary to maintain T cell unresponsiveness in a subject who has received an organ or bone marrow transplant to prevent graft rejection by inhibiting stimulation through CD2. In addition, T cell unresponsiveness can be maintained by blocking CD2 stimulation in a subject who has an autoimmune disease to alleviate symptoms of the autoimmune disease. In these cases, a CD2 inhibitory agent is administered to the subject in an amount and over a period of time sufficient to maintain T cell unresponsiveness. Alternatively, T cell unresponsiveness can be reversed in a subject bearing a tumor to stimulate a tumor-specific T cell response or in a subject receiving a vaccine to enhance the efficacy of the vaccine. For example, a cell (e.g., a tumor cell) can be modified to express a CD2 ligand or a CD2 stimulatory agent can be administered to the subject bearing a tumor or who has had a tumor surgically removed to prevent recurrence of th tumor. Additionally, antigen-specific responsiveness can be restored to anergized T cells in vitro by stimulating the T cells through CD2. Responsive T cells generated in vitro can then be administered to a subject.